1. Field of the Invention
This invention relates to erasing magnetic tape in high frequency data recording or video recording. More particularly, the invention relates to D.C. erasing magnetic tape with an asymmetric waveform.
2. History of the Art
There are three basic techniques that can be used to erase magnetic tape--A.C. erase, D.C. erase and bulk erase. Bulk erase refers to subjecting the tape to a damped sinusoidal waveform. When the sinusoidal waveform has died to zero amplitude, essentially zero remnant magnetic state is left in the magnetic tape. This is the most desirable erase condition for the magnetic tape. However, it is practical only for erasing entire reels of tape. In erasing blocks of information on tape, or portions of a tape, it is impractical to use a damped sinusoidal erase signal. This is due simply to the fact that the damped sinusoidal waveform would require a number of passes by the same head over the same area of the tape. Alternatively, multiple heads sequentially spaced one after the other might be used to create an equivalent damped sinusoidal erase signal for a given area of tape as it moved past the heads. Either of the above two techniques are unattractive from the standpoint of operating speed and cost.
D.C. erase of magnetic tape is probably the next most desirable erase technique after damped sinusoidal or bulk erase. The only apparent disadvantage of D.C. erase is that it does leave a D.C. remnant state in the magnetic tape. In the subsequent recordings, this D.C. remnant state may make it easier to record on the tape in one orientation rather than another orientation. As a result, distortion in recorded signals can occur due to time asymmetry created in the recorded signal by the relative hard or relative easy direction of magnetization during recording. This distortion is predictable and does not create noise problems when reading the magnetic tape.
Another problem associated with D.C. erase is that a D.C. signal cannot be passed across a transformer to a rotating magnetic head. In rotating head recording, signals to the magnetic head are typically coupled from a stator to a rotor carrying the head by use of a rotary transformer. A D.C. erase signal cannot be passed by a transformer. A prior solution to this problem is to place electronics on the rotor effectively generating the D.C. power on the rotor itself. An A.C. signal would be passed across a rotor and could be rectified and controlled to produce a D.C. signal. The D.C. signal generated on the rotor could then be used to drive a D.C. erase signal for a head on the rotor. While such an approach is feasible, it increases the cost and complexity of the rotor.
Obviously an A.C. erase signal could be used with a transformer to pass an erase signal to a rotating head. However, A.C. erase signals are probably the least desirable of the options in erasing magnetic tape. This is especially true at the high frequencies usually encountered with rotating head recording. For example, if the rotating head data frequency is nominally 10 MHz (Megahertz), then an A.C. erase signal should be in the order of 20-25 MHz. Further, this high frequency signal should have an amplitude such as to easily penetrate the full depth of the magnetic tape. These conditions place an abnormally high power requirement on driving the erase head.
Another potential problem with A.C. erase is that it leaves a low frequency residual in the magnetic tape after the erase is complete. In other words, the spectrum of the signal read from magnetic tape after an A.C. erase contains a hump of low frequency noise below a frequency 1/4 that of the erase frequency. The effect of this low frequency residual in the magnetic tape when reading subsequently recorded data signals is not easily predicted.
Therefore, it is the object of this invention to erase magnetic tape with a cyclic waveform that produces the effect of a D.C. erase.